Christians are called on to love their enemies, but also to oppose evil, especially when that evil threatens the innocent. That was why we rightly went to war against Nazi Germany (even though Germany had had a long history as a "Christian" nation).
I think that our national leadership ever since 9/11 has drawn a valid distinction between peaceful adherents of Islam (even though we as Christians might view those adherents as still in need of deliverance for the sake of their own eternal welfare from beliefs that we consider false), and those (such as ISIS) who would use Islam as a pretext for violent acts that are committed against both people and objects of cultural/historical significance; that are carried out even against other Muslims who do not share the perpetrators' radical beliefs; and that both Christians and peaceful Muslims would regard as evil.
In that respect, I believe that it is perfectly justified to consider ISIS in the same light as other similar entities or governments that we have fought against, and to oppose and resist them in whatever way we can (including the waging of armed conflict against them), wherever and whenever they arise.